Gram-positive microorganisms, such as members of the group Bacillus, have been used for large-scale industrial fermentation due, in part, to their ability to secrete their fermentation products into the culture media. In gram-positive bacteria, secreted proteins are exported across a cell membrane and a cell wall, and then are subsequently released into the external media usually obtaining their native conformation.
Secretion factors from Gram-positive microorganisms which have been identified and reported in the literature include SecA (Sadaie Y., Takamatsu h., Nakamura k., Yamane k.; Gene 98:101-105, 1991)., SecY (Suh J. -W., Boylan S. A., Thomas S. M., Dolan K. M., Oliver D. B., Price C. W.; Mol. Microbiol. 4:305-314,1990)., SecE (Jeong S., Yoshikawa H., Takahashi H.; Mol. Microbiol. 10:133-142, 1993), FtsY an FfH (PCT/NL 96/00278), and PrsA (WO 94/19471).
By contrast, in the gram-negative microorganism, E.coli, protein is transported to the periplasm rather than across the cell membrane and cell wall and into the culture media. E.coli has at least two types of components of the secretory mechanism, soluble cytoplasmic proteins and membrane associated proteins. Reported E.coli secretion factors include the soluble cytoplasmic proteins, SecB and heat shock proteins; the peripheral membrane-associated protein SecA; and the integral membrane proteins SecY, SecE, SecD and SecF.
In spite of advances in understanding portions of the protein secretion machinery in procaryotic cells, the complete mechanism of protein secretion, especially for gram-positive microorganisms, such as Bacillus, has yet to be fully elucidated.